On Mon, Oct 26, 2026 at 11:53 PM Michael S. Tsirkin <[email protected]> wrote: > The virtio_balloon driver reports a scatter-gather list of free pages > to the VMM so it may discard unused pages. > > For an arm64 kernel with 64 KiB pages, the default reporting and > allocation orders allow entries ranging from 2 MiB to 512 MiB. If the > report contains a sufficient number of large entries, we will end up > with a descriptor chain that represents over 4 GiB of memory. This > violates virtio spec, which says: > Drivers MUST NOT add a descriptor chain longer than $2^{32}$ bytes in > total; > > Specifically, submitting $2^{32}$ or more breaks Rust-VMM. > > Submit each reported page as a separate virtqueue buffer instead of one > buffer with an sg list of all pages. This also avoids indirect > descriptor allocation (kmalloc in the reporting path). > > To avoid multiple buffers causing interrupt storms, keep callbacks > disabled while we wait for an event. > > TODO: as an alternative, keep using virtqueue_enable_cb_delayed > but add buffers after there's space in the ring. > > Note: if the virtqueue is broken, wait_event on > virtqueue_get_buf hangs. This is a pre-existing issue: > the old single-buffer code had the same hang. > > Reported-by: Ruben Hakobyan <[email protected]> > Cc: Ruben Hakobyan <[email protected]> > Fixes: b0c504f15471 ("virtio-balloon: add support for providing free page > reports to host") > Cc: Alexander Duyck <[email protected]> > Signed-off-by: Michael S. Tsirkin <[email protected]> > --- > > Notes (changelog): > changes since v1: > keep callbacks disabled while draining: get_buf re-arms the used > event for the next buffer, reintroducing an interrupt per page. > Disable the reporting vq callbacks at probe, and only arm the > delayed callback to wait. >
Thanks for the patches! I think sashiko-bot found some issues on broken queues in the first patch that are worth looking into for correctness. Other than that, I tested the "normal path" on QEMU and Cloud Hypervisor, both on 4 KiB and 64 KiB guests, and this works correctly.
